A problem with currently available tobacco smoke filters, particularly cigarette filters, is the difficulty of disposing of such materials after use. With limited exceptions, cigarette filters are presently formed from highly crimped cellulose acetate fibers bonded at their contact points to provide a significant volume of interstitial space for the passage of smoke. The bonded contact points of such filter elements degrade very slowly under normal environmental conditions resulting in high volume, long life, environmentally undesirable litter. The cellulose acetate fibers themselves are, for all intents and purposes, effectively not biodegradable.
Certain polymeric materials such as polyvinyl alcohol and ethylene vinyl alcohol copolymers are known to readily soften or dissolve in the presence of water. Berger U.S. Pat. No. 5,509,430, the subject matter of which is incorporated herein in its entirety, discloses the production of tobacco smoke filters from bicomponent fibers utilizing a sheath of polyvinyl alcohol or ethylene vinyl alcohol copolymer and a core of a thermoplastic polymer such as polypropylene. The sheath-forming materials of the bicomponent fibers used to make tobacco smoke filters according to the No. '430 patent will dissolve in the presence of environmental moisture, degrading the bonds between the fibers and resulting in the collapse of the matrix structure, while leaving behind a multiplicity of non-degraded core fibers.
Thus, while filter elements utilizing bicomponent fibers according to the No. '430 patent provide a substantial improvement in reducing the bulk of environmental litter from tobacco smoke filter elements, it would obviously be desirable to be able to produce tobacco smoke filter elements wherein the fibrous matrix is substantially entirely biodegradable. Yet, current technology has been unable to resolve the inherent conflict in producing a product that will substantially totally disintegrate in the presence of environmental water when it has served its purpose, while retaining its structural integrity during and after a manufacturing process which, of necessity, takes place in the presence of moisture.
Polyvinyl alcohol is unique in being the only biodegradable, carbon-carbon backbone polymer that can completely biodegrade to small molecules, e.g., carbon dioxide and water, under conditions found in typical waste treatment facilities and under soil burial conditions. Polyvinyl alcohol is commercially available as a thermoplastic, water soluble, solid polymer that is relatively inexpensive and non-toxic. Conventionally, polyvinyl alcohol is made by hydrolyzing polyvinyl acetate to selectively remove acetate groups from the polymer chain and replace the acetate groups with hydroxyl groups. Generally, complete or near complete conversion increases the crystallinity of the polymer resulting in polyvinyl alcohol which is more difficult to completely dissolve in cold water Intermediate hydrolysis grades (those less than 90% and greater than 65%) exhibit good cold water solubility and allow a product made therefrom to disintegrate rapidly in the presence of environmental moisture or at a waste disposal site. In contrast, fully hydrolyzed grades (those greater than 98%) impart good strength in cold water, but do not dissolve rapidly upon disposal. By varying the chain length of the polyvinyl acetate starting material and controlling the hydrolysis using well known techniques, polyvinyl alcohol having carefully selected properties can be realized. This invention is primarily concerned with polyvinyl alcohol starting materials having less than about 95%, optimally less than 90%, and more than about 65%, of its acetate groups replaced by hydroxyl groups.
Filtration efficiency, i.e., the ability to remove undesirable constituents from tobacco smoke, is obviously one of the most important properties of materials used in the production of tobacco smoke filters. Heretofore, high levels of filtration have often had to be compromised in order to satisfy other commercially important factors such as impact on taste, resistance to draw, hardness and manufacturing costs. Tobacco smoke filters formed of polyvinyl alcohol fibers have been found to provide excellent filtration efficiency and commercially acceptable taste properties. However, because of the hygroscopicity of polyvinyl alcohol, prior art attempts to process such materials have resulted in filter rods which are too soft to be efficiently handled in high speed cigarette manufacturing machines. Moreover, cigarette filter elements formed from such materials have failed to provide a sufficiently stable porous matrix to permit proper draw characteristics and avoid collapse in use.
While bicomponent fibers comprising a thin sheath of polyvinyl alcohol supported by a relatively non-degradable core such as polypropylene, as described in the aforementioned No. '430 patent, can be processed into a relatively self-sustaining porous rod useful in the production of tobacco smoke filters and the like, attempts heretofore to utilize fibers formed substantially entirely of polyvinyl alcohol have been unsuccessful. The instant invention, utilizing unique technology, has overcome these problems, enabling the high speed production of substantially totally biodegradable tobacco smoke filter elements that can be readily processed by state of the art cigarette-making machines with commercially acceptable hardness and resistance to draw, good taste, and filtration efficiency (total particulate matter retention) equal to or better than currently available tobacco smoke filters formed from cellulose acetate tow.
As discussed in the aforementioned No. '430 patent, it is known that filtration efficiency can be increased through the use of fine fibers which provide increased surface area at the same fiber weight. Solvent-spun cellulose acetate fibers are commercially available only in fiber sizes down to 13 microns in diameter. To obtain finer cellulose acetate homopolymer fibers, e.g., 10 microns or less, melt spinning of plasticized cellulose acetate resin would be required; however, the level of plasticizer necessary to directly spin such fine cellulose acetate fibers would render the resultant fibers very weak and commercially useless. Melt spun cellulose acetate fibers of a larger diameter, which would require less plasticizer, would have to be drawn and crimped to produce such fine fibers for use in tobacco smoke filters. Unfortunately, melt spun cellulose acetate fibers can only be commercially drawn at relatively low draw ratios before the fibers break during processing. The inability to form and process very fine fibers of cellulose acetate places practical limits on the filtration efficiency capabilities of this material in the production of tobacco smoke filters.
One solution to this problem is the use of the bicomponent fibers disclosed in the No. '430 patent since such fibers, even with a cellulose acetate sheath, can be melt blown to produce very fine fibers, on the order of 10 microns or less, even as low as 1 micron. In the melt blowing process, a molten thermoplastic polymer is forced through a spinnerette or the like to form a multiplicity of continuous fibers. As the molten fibers exit the die, they are impacted by a high velocity stream of hot primary gas, such as air, that mixes with ambient air, resulting in a turbulent stream that rapidly attenuates and solidifies the fibers which are collected in an entangled mass or web.
Research at the University of Tennessee has shown that fibers consisting essentially of partially hydrolyzed (less than 90%) polyvinyl alcohol can be melt blown successfully. See "Development and Evaluation of Water Soluble Melt Blown Nonwovens", Maureen Deever, Roberto S. Benson and Nancy Fair, INDA JNR, Volume 5, No. 2, pp. 27-33, incorporated herein in its entirety. The University of Tennessee research was principally directed to the production of non-woven fabrics formed of melt blown polyvinyl alcohol fibers. The INDA JNR article suggests two chemical treatments for moderating the water solubility of such materials in order to make them useful for such applications: 1) mixing a fluorochemical melt additive with the polyvinyl alcohol resin before melt blowing, or 2) applying an organic solvent-based water-repellent finish to the polyvinyl alcohol non-woven material as a post-treatment. Both methods reduce the rate of cold water solubility of the pure polyvinyl alcohol melt blown material, neither process would be acceptable in the commercial production of tobacco smoke filters from polyvinyl alcohol fibers.
Another technique for forming a non-woven web of fine polyvinyl alcohol fibers, from 0.1 to 30 microns in mean fiber diameter, is disclosed in Rhim U.S. Pat. Nos. 5,342,335 and 5,445,785, the disclosures of each of which are incorporated herein in their entirety by reference. Rhim extrudes polyvinyl alcohol fibers from an aqueous solution, rather than a melt, attenuates the fibers with a gaseous source, dries the attenuated fibers, and then deposits them randomly on a moving foraminous surface much like a melt blowing process.
While solution spinning and attenuation of polyvinyl alcohol to produce fine fibers can provide an acceptable starting material for the process of the instant invention; melt blowing extruded polyvinyl alcohol fibers is preferred. Various prior art melt blowing processes and appartus can be used for this purpose. For example, reference is made to Buntin, U.S. Pat. Nos. 3,595,245 and 3,615,995, Schwarz, U.S. Pat. Nos. 4,380,570 and 4,731,215, Lohkamp et al., U.S. Pat. No. 3,825,379, and Jezic U.S. Pat. No. 5,021,288, the entire subject matter of each of which is incorporated herein for further background in this technology.
The method of manufacturing the polyvinyl alcohol polymer or the fiber used in the production of tobacco smoke filters is not part of the instant invention, except as described herein. Processes for making polyvinyl alcohol polymers and for forming fibers therefrom are well known in the art and most commercially available polyvinyl alcohol fibers can be used if processed according to the instant inventive concepts. Thus, while increased surface area resulting from attenuating polyvinyl alcohol fibers as they are extruded in a melt blowing or solution spinning process provides better filtration properties in the final product, according to the broader concepts of this invention the polyvinyl alcohol fibers can be melt spun or spun bonded or otherwise formed in accordance with any conventional and well-known fiber-forming techniques. Consistent therewith, for example, melt spun or spun bonded polyvinyl alcohol fibers having an average diameter of from about 10 to 30 microns may desirably be used to form tobacco smoke filters according to this invention, although melt blown or solution spun polyvinyl alcohol fibers having an average diameter of about 10 microns or less are particularly advantageous because of their high surface area.
While tobacco smoke filters formed of fibers consisting entirely of polyvinyl alcohol homopolymer are unique and commercially desirable, the polyvinyl alcohol polymer used in the formation of the fibers may include some quantities of other substances inherent in such materials and/or additives necessary or desirable to facilitate commercial production requirements. Reference herein and in the accompanying claims to the continuous fibers from which the porous matrix of the tobacco smoke filters of the instant invention are formed as "consisting essentially of polyvinyl alcohol" is intended to encompass the presence of small amounts of other materials which do not significantly affect the nature or function of the polyvinyl alcohol in the formation of porous elements, or their use as tobacco smoke filters, according to this invention.
Likewise, while it may be highly desirable to produce tobacco smoke filter elements wherein the porous matrix is formed entirely of polyvinyl alcohol fibers, minor proportions of other polymeric fibers or materials, including cellulose acetate fibers, may be incorporated for special applications. However, to achieve the significant advantages of the instant invention, particularly the biodegradability of the bonds that cause the collapse of the filter matrix in the presence of environmental moisture, and, desirably, the degradation of a major proportion, or even substantially all, of the matrix itself, tobacco smoke filters according to this invention should comprise at least 50% by weight, and preferably 70 to 90% by weight, of fibers consisting essentially of polyvinyl alcohol.
To even further reduce environmental litter, if an overwrap is necessary for integrity, the porous rod used in the formation of tobacco smoke filter elements according to this invention can be encased in a thin sheath of polyvinyl alcohol in lieu of conventional paper plug wrap, thereby enhancing the biodegradability of the entire product.
Various other properties of such filters can be modified or improved by the incorporation of liquid or solid additives during the manufacturing process. For example, non-aqueous flavor-modifying materials such as menthol may be sprayed onto the fiber as it is formed, to provide a menthol flavor to tobacco smoke passing through a filter element in a cigarette or the like incorporating the same.
Fine activated charcoal particles may be added to a web or roving of such fibers, preferably during a melt blowing process, but in any event, before gathering and forming the same into a filter rod, to provide gas phase filtration characteristics to the resulting filter element. Likewise, chopped cellulose acetate fibers or even tobacco particles, can be randomly dispersed in the porous matrix for enhanced taste and other characteristics.
Particles of a biodegradable additive such as a starch, may also be incorporated in a similar manner to increase the hardness of the filter rod, if necessary. Sorghum is a particularly useful starch since it is water soluble, 100% biodegradable, non-toxic, light weight, and rodent and insect neutral. It is also inexpensive, sorghum meal, available from Archer, Daniels, Midland, selling for approximately 25% of the cost of corn starch. Other sources of sorghum useful in the instant invention are packaging materials, such as Envirofill manufactured by Norel, a division of UniSource, and Bifpak available from Virginia Biofoam Co.
Expanded sorghum, produced by treating sorghum meal with heat and pressure and releasing the product through a narrow opening increasing the volume 50-fold, is available from Biofoam. It has been found that such material, when ground and incorporated into the rod-forming web according to this invention, can increase the filtration efficiency of a tobacco smoke filter made therefrom by an additional 5%.
By incorporating a particulate additive in the gas stream of a melt blowing attenuation, uniform dispersion and excellent adherence to the softened surface of the polyvinyl alcohol fibers is effected, reducing the necessity for an additional filter element as is commonly used to preclude loosely adhering additive particles from being drawn into a smoker's mouth, although a filter plug according to the instant invention may be incorporated with other plugs in a multi-filter cigarette for other reasons, if desired.
The porous rod-like elements of this invention may have other applications; however, their use in the formation of tobacco smoke filters is of particular interest. Moreover, while such tobacco smoke filter elements may be associated with cigarettes, cigars or pipes, the primary commercial application of such products relates to their use as filters for cigarettes. Therefore, the following description of the instant inventive concepts will focus on the production of tobacco smoke filter elements for use in filtered cigarettes as exemplary of the broader applications for this invention.